Method of producing castings having high mechanical properties



United States Patent C METHOI) OF PRODUCING CASTINGS HAVING HIGHMECHANICAL PROPERTIES Lucien Jules Pras, Billancourt, France, assignorto Regie Nationale des Usines Renault, Billancourt, France No Drawing.Application July 7, 1954, Serial No. 441,923

Claims priority, application France August 27, 1953 Claims. (Cl.148-211) This invention relates to a method of producing castings havinghigh mechanical properties.

It is known that in malleable iron castings, that is to say white ironcastings treated thermally for the purpose of graphitisation, thegraphite is obtained in nodular form, i. e., in the form of nodes orpiles of very irregular form, and that owing to this fact the mechanicalproperties are appreciably lower than those of iron castings in whichthe graphite is spheroidal, this being obtained in direct fashion uponcasting in the case of compositions whch are either hypereutectic,comprising cerium, or which contain magnesium, usually in associationwith nickel.

it is known that sulphur interferes with the graphitisation of malleableiron castings, and that its effect must be neutralised by an excess ofmanganese, in which connection the ratio of Mn/S must be greater than 3.

Since an excess of manganese may interfere with the graphitisation, themanganese must be limited, and in consequence the positionis similarwith regard to sulphur, which usually remains at less than 0.15%.

It is furthermore known that sulfur lowers the mechanical properties ofsteel, so that with the exception of ordinary screw cutting steelscontaining manganese sulphi'cle the lowest percentages of sulphur areusually sought, generally below 0.05%.

it is, therefore, paradoxical .to endeavour to improve the properties ofiron castings by a substantial increase in the sulphur with acorresponding reduction in manganese, It is accordingly a matter ofsurprise thus to arrive at mechanical properties which have not beenobtained heretofore in graphitised White iron castings or, moregenerally, in iron castings free from alloyed substances. These meansand the new and surprising results obtained represent .theessentialfeatures of the present invention.

It has been made known heretofore that when the proportion of S/Mn in awhite iron casting is increased the graphitisation upon annealing tendsto change the graphite of nodular form in irregular .heaps into .a

spheroidal form, in which the spherules are more and more symmetricaland regular. This scientific fact, however, has not heretofore acquiredindustrial significance, because the casting also comprised lamellargraphite, formed upon solidification, or irregular nodular graphitedeveloped during annealing or, if only spheroidal graphite was involved,it was impossible to obtain complete transformation of the cementitecaused by solidification, irrespective of the duration of the annealingprocess.

In each case the mechanical properties remained poor.

It is known that in malleable white ,iron castings a martensitichardening facilitates fine graphitisation during the subsequentannealing process. .It has shown previously, as a result ofinvestigations, that this treatment reaches its full effectiveness whenit is followed by a .graphite crystal nucleus forming treatment ,ataround 450 and of sufficient duration (10-50 .hours) before Patented May7, 1957 graphitisationis effected by annealing at high temperature.

In the sulphur castings forming the subject matter of the presentinvention the white iron obtained upon the pouring operation must alsobe subjected to a hardening process before annealing is performed forthe purpose of graphitisation. However, in contradistinction to thepreceding case, the crystal nuclear forming treatment should not becarried out, as it is not the object to multiply to a maximum thegraphite nucleicapable of giving rise to fine nodules, but more probablyto bring about nuclear formation of the graphite in the fine inclusionsof iron sulphide.

According to the invention it is proposed to employ an iron casting richin a finely distributed iron sulphide (FeS) content; to effect amartensitic hardening, the martensite formedgiving rise by heating tounstable percarbides which are capable of subsequent conversion intocementite and graphite according to a reaction of the type:

then to form the graphite nuclei by epistaxis on-the hexagonal pyrites,and finally topermit free .development of the nuclei to provide graphitespherules.

It maybe remarked that if the formation of nuclei is concerned, the solecondition of similarity of the crystalline system suffices, a t-elationbetween the lattice dimensions not being necessary.

The invention, which permits in the case of cast iron, which is totallywhite upon the pouring operation, of a graphitisation upon annealingwithout the formation of cementite produced by solidification, resultingin a totally spherulitic graphite, is characterised by the followingfeatures:

(1) The composition comprisesa proportion of S/Mn which is greater than1and more particularly less than 0.15 manganese and more than 0.2sulphur, and preferably it is comprised within the following limits:

C=2.l0 to 2.70 Si=0.85 to 1.10 Mn=,0.002 to 0.15 S=0.20 to 0.30 P=0.l0

Generally speaking, there are not special components.

(2) solidification in the mould-should take place as rapidly aspossible, and preferably more rapidly than is strictly necessary toobtain the white structure. This leads to ,afinedistribution of theironsulphide, on which there depends the fineness ofdistribution-of thegraphite spherules obtained during the course of the subsequentannealing process. Casting in a chill mould is preferable to casting insand.

(3) The thermal treatment comprises essentially a martensite hardeningvor, if desired, ,an inferior hardening before carrying out thegraphitisation by annealing.

This hardening is performed with advantage in conditions of rapidcooling, preferably for example in oil, or if desired in a bath ofsalts, which may be a fusible mixtureof alkaline nitrate and alkalinenitriteand leads to a martensitic structure.

To avoid the danger of shrinkage upon hardening there has beenrecognised according to the invention the prime importance ,of effectinga preliminary annealing, .a simple austenisation beyond termination ofthe transformation upon the heating A1 followed by cooling at a mediumspeed,-such as cooling in still .air.

According to theinvention is has also been. recognized that if thecooling for the purpose of solidification is moderate (casting insand),it.rnay benecessary to carry out two consecutive.austenisation"treatments with marnealing beyond the point A1, i. e., ata temperature sufiiciently high so that the casting will be constitutedonly by cementite and austenite, such temperature being maintained for asufficient length of time to permit of total disappearance of thecementite not dissolved in the austenite at the annealing temperature.The duration of this annealing process is less than that in the case ofusual white iron malleable castings annealed at the same temperature.

The consecutive cooling is carried out in accordance with the mechanicalproperties sought to be achieved. There may be obtained a totaleutectoidal graphitisation in respect of a ferrite and graphitestructure, as in malleable castings of the usual kindvery softorpreferably the graphitisation may be stopped at such a point that thematrix has a substantially eutectoidal (lamellar perlite) composition,which leads to increased toughness.

Finally, it is possible as a subsequent step to carry out austenisation,quenching and tempering in order to obtain a greater toughness.

An example of reduction to practice of the method according to theinvention is given in the following:

Pieces of metal having a weight of 700 grams and a mean thickness of 20mm. are cast in moulds which are partially metal and partially composedof sand.

The castings have the following composition:

Carbon=2.5 Silicon=l Manganese=0.l2 Sulphur=0.28 Phosphorus=0.l

Following stripping and complete cooling of the castings the latter areheated to 820 over a period of 30 minutes and are cooled in still air.There is thereupon carried out a second heating to 825 over a period of30 minutes, and the pieces of metal are hardened in oil. The pieces arethen reheated to 940 for a period of eight hours, after which they areremoved from the furnace and cooled in still air.

A structure is obtained comprising fine graphite spherules on a lamellarperlite base.

Mechanical tests carried out on test pieces taken from these metal partshave produced the following results:

Tensile strength: 55 to '75 kg./mm. Breaking strength: 55 to 100 kg./mm.Elongation limit: 5 to 6% Properties of this nature have not beenobtained collectively heretofore in conjunction with castings devoid ofall alloyed substances.

The limit of resistance to rotative bending has been measured inconjunction with Moore test pieces.

The resistance limit has been found to be 38 to 40 kg./mm. which isgreater than that of the usual spheroidal castings.

The method referred to has the following advantages:

It is applicable to readily produced castings which make use of orescontaining sulphur, and are of low commercial value, and it does notresort to any special substance alloyed therewith.

It provides, upon annealing, a very even distribution of graphite in theform of spherules, this structure being associated itself with very highmechanical properties, the breaking strength being between 95 and 105kg./mm. with an elongation of between 5 and 6.5% if the matrix islamellar perlitic.

Its industrial application is economical and it can be readily utilised,leading to uniform nature of the results obtained.

What I claim is: p

1. A process of thermallytreating white castings to produce malleableiron with annealed spheroidal graphite therefrom, said castingscontaining 0.2 to 0.3% sulfur and 0.002 to 0.15 manganese with the ratioS/Mn being greater than 1, said process comprising the steps of heatingsaid castings and directly quenching to impart a martensitic structureto the metal, and graphitization annealing at a temperature higher thanthat employed in the heating step, said temperature being maintaineduntil total disappearance of the cementite not dissolved in theaustenite at the annealing temperature.

2. A process as defined in claim 1, wherein said castings also contain2.1 to 2.7% carbon, 0.85 to 1.1% silicon and 0.1% phosphorus.

3. A process of thermally treating white castings to produce malleableiron with annealed spheroidal graphite therefrom, said castingscontaining 0.2 to 0.3% sulfur and 0.002 to 0.15 manganese with the ratioS/Mn being greater than 1, said process comprising the steps of heatingsaid castings and directly quenching to impart a martensitic structureto the metal, and graphitization annealing at a temperature of 900 to950 C. which is higher than that in the heating step, said annealingtemperature being maintained for a period of 5 to 12 hours to effecttotal disappearance of the cementite not dissolved in the austenite atthe annealing temperature.

4. A process of thermally treating white castings to produce malleableiron with annealed spheroidal graphite therefrom, said castingscontaining 0.2 to 0.3% sulfur and 0.002 to 0.15 manganese with the ratioS/Mn being greater than 1, said process comprising the steps of heatingsaid castings to a temperature of about 820 C. for about thirty minutes,cooling in air, heating again to a temperature of about 825 C. for aboutthirty minutes, hardening in oil, reheating to about 940 C. for abouteight hours, and cooling in air.

5. A process as defined in claim 1, wherein said castings are formed insemimetallic molds.

6. A process of thermally treating white castings to produce malleableiron with annealed spheroidal graphite therefrom, said castingscontaining 0.2 to 0.3% sulfur and 0.002 to 0.15 manganese with the ratioS/Mn. being greater than 1, said process comprising the steps ofelfecting a preliminary annealing of the castings and cooling in air,heating the thus annealed castings and directly quenching to impart amartensitic structure to the metal, and graphitization annealing at atemperature higher than that employed in the heating step, saidtempenature being maintained until total disappearance of the cementitenot dissolved in the austenite at the annealing temperature.

7. A process of thermally treating white castings to produce malleableiron with annealed spheroidal graphite therefrom, said castingscontaining 0.2 to 0.3% sulfur and 0.002 to 0.15 manganese with the ratio5/ Mn being greater than 1, said process comprising the steps of heatingsaid castings and directly quenching to impart a martensitic structureto the metal, repeating said heating and said quenching, andgraphitization annealing at a temperature higher than that employed inthe heating step, said temperature being maintained until total disappearance of the cementite not dissolved in the austenite at theannealing temperature.

8. A process of thermally treating white castings to produce malleableiron with annealed spheroidal graphite therefrom, said castingscontaining 0.2 to 0.3% sulfur and 0.002 to 0.15 manganese with the ratioS/Mn being greater than 1, said process comprising the steps of heatingsaid castings and directly quenching to impart a martensitic structureto the metal, graphitization annealing at a temperature higher than thatemployed in the heating step, said temperature being maintained untiltotal disappearance of the cementite not dissolved in the austenite atthe annealing temperature, and cooling said castings in air.

9. A process of thermally treating white castings to produce malleableiron with annealed spheroidal graphite therefrom, said castingscontaining 0.2 to 0.3% sulfur and 0.002 to 0.15 manganese with the ratioS/ Mn being greater than 1, said process comprising the steps of heatingsaid castings and directly quenching to impart a martensitic structureto the metal, graphitization annealing at a temperature higher than thatemployed in the heating step, said temperature 'being maintained untiltotal disappearance of the cementite not dissolved in the austenite atthe annealing temperature, and quenching said castings.

10. A process of thermally treating white castings to produce malleableiron with annealed spheroidal graphite therefrom, said castingscontaining 0.2 to 0.3% sulfur and 0.002 to 0.15 manganese with the ratio8/ Mn being [greater than 1, said process comprising the steps ofheating s aid castings and directly quenching to impart a martensiticstructure to the metal, graphitization annealing at a temperature higherthan that employed in the References Cited in the file of this patentUNITED STATES PATENTS 1,542,440 Drysdale June 16, 1925 1,932,664 Hayeset al. Oct. 31, 1933 2,008,452 Iewett et a1. July 16, 1935

1. A PROCESS OF THERMALLY TREATING WHITE CASTINGS TO PRODUCE MALLEABLEIRON WITH ANNEALED SPHEROIDAL GRAPHITE THEREFRO, SAID CASTINGS CONTAINIG0.2 TO 0.3% SULFUR AND 0.002 TO 0.15 MANGANESE WITH THE RATIO S/MN BEINGGREATER THAN 1, SAID PROCESS COMPRISING THE STEPS OF HEATING SAIDCASTING AND DIRECTLY QUENCHING TO IMPART A MARTENSITIC STRUCTURE TO THEMETAL, AND GRAPHITIZATION ANNEALING AT A TEMPERATURE HIGHER THAN THATEMPLOYED IN THE HEATING STEP, SAID TEMPERATURE BEING MAINTAINED UNTILTOTAL DISAPPEARANCE OF THE CEMENTITE NOT DISSOLVED IN THE AUSTENITE ATTHE ANNEALING TEMPERATURE.